1. Field of the Invention
The present invention relates to the use of a clamp-and-strobe differential amplifier circuit in a bubble memory sense amplifier. Specifically, the present invention relates to a constant voltage biasing circuit, an AC coupling circuit and a differential comparator for detecting changes in resistance of bubble memory magneto-resistive detectors.
2. Description of the Prior Art
Conventional clamp-and-strobe bubble memory sense amplifier circuits have biasing circuits for establishing a current through the bubble memory detectors and have capacitors for a capacitively coupling a preamplifier to a comparator and clamp circuit. However, these circuits typically have fixed current biasing circuits which results in a wide variation in the d.c. operating voltage applied to the bubble memory detectors. Since the amplitude of the bubble signal is a function of the voltage across the detectors, it is desirable to have a constant d.c. operating voltage independent of the resistance of the detector to produce the same signal amplitude from any bubble memory.
Typically, bubble memory sense preamplifiers use a clamp and strobe technique to eliminate low frequency clock noise. Characteristically, this low frequency clock noise is quite large in comparison to the voltage variation caused by the bubble signal. The clamp and strobe technique clamps the output voltage of the preamplifier to ground until the anticipated bubble signal occurs. The output is then allowed to float and capacitively coupled to the comparator. This results in a DC bias being stored on the capacitors which cancels the DC noise at the time the clamp signal is released and allows the voltage variation of the small bubble signal to be sensed. However, this technique requires a capacitive coupling between the amplifier and the comparator and a clamp circuit coupled between the capacitive coupling and the comparator, which places an undesirable capacitive load on the amplifier output and is difficult to implement in an integrated circuit.
Bubble memories typically are characterized by bubble signals having a frequency between 500 kHz-2 mHz. A wide band preamplifier is desired in order to be compatible with the wide range of bubble memory types. The coupling capacitor between the preamplifier and the comparator needs to be around 100 pF to provide this -0.1 db band width. In addition, the amplifier must be stable when the capacitors are clamped to ground by the clamp circuit. Unfortunately, capacitors in the 100 pF range are typically quite large when implemented on semiconductor circuits. Further, it is difficult to compensate AC coupled wide-band amplifiers for large capacitive loads on their output. Specifically, large capacitors on semiconductor circuits typically are constructed from metal-oxide-N.sup.30 capacitors known as "Metal Capacitors." These metal capacitors are characterized by their large size, typically 1500 sq. mils for a 100 pF capacitor, and by a large parasitic capacitance coupled to ground. Typically, this parasitic capacitance is of approximately the same order of magnitude as the coupling capacitance. When such metal capacitors are used in the present circuit they result in a 100 pF parasitic capacitance between the output of the preamplifier circuit and ground. Such a capacitive loading at the output causes a lag in the output signal. Since most preamplifier circuits utilize feedback, this lag may result in an instability at high frequencies. To compensate for this instability, the high frequency gain must be reduced by increasing the size of a compensation capacitor. However, this reduces the bandwidth of the preamplifier. It is undesirable to reduce the bandwidth of the preamplifier or to use a large compensation capacitor which takes up an undesireably large amount of space on a semiconductor. Secondly, it is desirable to lower the capacitive loading on the output of the preamplifier circuit and to use physically smaller coupling capacitor while simltaneously maintaining the capacitive coupling between the preamplifier and the comparator at the 100pF level. By lowering the capacitor loading on the preamplifier, assuming that the coupling capacitance can be left at the same value, a higher-band width can be attained, and by making the capacitor physically smaller, valuable area on the semiconductor die can be saved. In addition, it is desirable to have a differential output from the preamplifier but this makes it difficult to compare the output signal to a threshold voltage. Accordingly, it is desirable to provide constant d.c. voltage biasing circuit, an improved coupling circuit characterized by a smaller loading on the preamplifier output, and a differential comparator adapted to compare a differential signal with a threshold voltage.